1. Field of This Invention
This invention relates to a method and an arrangement of apparatus to shift the frequency of a monochromatic light beam, especially a laser beam, by means of the principle of the Doppler effect using reflection from moving mirrors. In particular, the apparatus arrangement represents a mechanical laser frequency converter with wide frequency sweep.
For numerous applications (e.g., spectroscopy, photochemistry, and isotope separation) radiation of a defined wavelength is required. There are certain spectral regions where tunable radiation is available from appropriate lasers, for example, dye-lasers. In some cases tuning can be a effected by using non-linear optical effects such as induced Raman scattering and parametric oscillators. In other spectral regions such tunability does not exist or the tuning range is too small.
It is known that a frequency shift occurs when light is reflected from a single mirror moving in any direction except parallel to its surface. However, any other steady movement of the mirror also changes the beam direction or leads to a parallel displacement of the beam.
Furthermore, there is a known apparatus arrangement taking advantage of the Doppler effect in which the laser beam is directed into a defined resonator by means of two or three mirrors. After a certain number of reflections and beam leaves the resonator traversing a similar system of mirrors. In such case the resonator consists of a flat mirror located at the periphery of a rotating circular cylinder and a spiral reflecting cylinder having the shape of the involute of the circular cylinder (see T. J. Manuccia, "CW IR Laser Induced Chemistry, Isotope Separation and Related Laser Technology at NRL", in "Laser in Chemistry", Michael A. West (ed.), Elsevier Scientific Publishing Company, Amsterdam, (1977), pages 210-215). Manuccia proposed the use of such an apparatus arrangement for the very important application of isotope separation by tuning the wavelength of available lasers to the most favorable absorption frequency.
Taking a displacement in the spiral of .pi..multidot.20 cm, 60,000 rpm and 20 successive resonator passages of the same beam as the basis, the frequency shift amounts to 2.5 GHz at 10 .mu.m.
To that extent such known arrangement could meet the experimental requirements, according to absorption wavelength and available lasers. However, there are considerable drawbacks from a technical point of view. A diameter of the inner cylinder of 20 cm (displacement of the spiral .pi..multidot.20 cm) implies an outside diameter of the spiral cylinder of 1 m and a weight of 1 to 2 tons. Therefore, the system is heavy, bulky and awkward to use. The spiral reflector can only, if at all, be produced with the necessary precision at the expense of very high costs.
In addition, it has been shown and experimentally investigated that a Doppler effect occurs if an echelon grating instead of a mirror is moved along its surface [J. Bahrmann, K. R. Detring, and G. Simonsohn, "Optics Communications" 22, 3 (1977), pp. 365-368]. However, a grating exhibits comparatively large reflection losses resulting in only a small number of reflections and thus a relatively low overall effect.